Low density foam, such as polystyrene foam, is commonly made by combining a physical blowing agent with a molten polymeric mixture under pressure and, after thorough mixing, extruding the combination through an appropriate die into a lower pressure atmosphere.
From about the 1950's to the present, physical blowing agents of choice have included halocarbons, hydrocarbons or combinations thereof. Examples of these include commercially available halocarbon compositions such as dichlorodifluoromethane, trichlorofluoromethane and mixtures thereof, and the C.sub.2 -C.sub.6 hydrocarbons.
During the 1980's, the worldwide scientific community presented sufficient evidence linking chlorofluorocarbons (CFCs) with atmospheric ozone depletion and sought governments to regulate CFC's. As of a result of such regulations, hydrocarbons are generally the choice of physical blowing agents. Hydrocarbons, especially the short-chained hydrocarbons, produce foams with satisfactory physical properties, but with the adverse effect of flammability. Flammability of the foam product is typically reduced during an aging process that allows the hydrocarbons to be released. This aging process that reduces the flammability of the foam to a safe level depends on factors such as the rate of release (which is dependent on factors such as the foam structure and the selected blowing agent) and the amount of hydrocarbons or the like initially remaining in the produced foam.
There are two foams which are commonly produced. The first foam is made from polystyrene and the second foam is made from low density polyethylenes (LDPEs). The pure polystyrene foam is too brittle for some applications such as protective packaging which require protection from multiple impacts.
LDPE foams are generally considered to be resilient and non-brittle, which are desirable properties. The LDPE foams, however, have a disadvantage in that a stability control agent (also referred to as a permeation modifier) needs to be added to the polymeric composition to produce a commercially acceptable foam (e.g., a foam that does not change its dimensions significantly over time). The stability control agent, however, has a negative effect on the foam in that it prolongs the retention time of the physical blowing agent in the foam.
The amount of total residual blowing agent in the LDPE foam immediately after its manufacture is typically in the range of from about 5 to about 10 weight percent of the polymeric composition. This amount is dependent upon factors such as the desired density of the foam and the selected blowing agent. This amount of total residual blowing agent generally produces a potentially flammable condition if the foam is located in a confined area. Typically, the aging process for a LDPE foam containing a stability control agent takes from about 14 to about 30 days. The aging process is dependent upon a number of factors including, but not limited to, the density of the foam, the selected blowing agent and storage temperature of the foam.
It is an object of the invention to reduce the flammability of foam associated with blowing agents such as hydrocarbons or some partially fluorinated organic blowing agents in post-production usage applications.
It is another object of the invention to reduce the aging requirement associated with, for example, hydrocarbons or some partially fluorinated organic blowing agents.
It is yet another object of the present invention to reduce the degree of collapse of the produced foam caused by the diffusion of a physical blowing agent(s) from the foam cells after the foam achieves its structural form.
It is a further object of the present invention to produce a foam that can be processed with conventional foaming machinery.
It is yet a further object of the present invention to produce a foam that is light in weight.
Lastly, it is object of the invention to produce a resilient, non-brittle foam structure suitable in applications such as protective packaging of delicate and fragile high surface quality goods.